The invention relates to refractometric devices for measuring the concentrations of solutions and, more particularly, to immersion-type refractometric devices.
Refractometric devices which are immersed in a solution to be measured and employ the principal of internal reflection and external refraction of light are well known. In one type refractometric device, a prism having a larger refractive index than that of the solution is placed in the solution, a light source beams rays into the boundary surface between the solution and the prism, and the incident light reflected from the boundary surface is received by a photosensitive device which provides an output signal indicative of the solution concentration in response to the intensity of the reflected light. This type device usually is quite bulky, thereby limiting its field of application, and/or is expensive to manufacture. U.S. Pat. Nos. 2,780,131, 2,807,976 and 3,751,672 are representative prior art references disclosing this type refractometric device.
Another type refractometric device includes a probe having a light-transmissive, rod-like body (e.g. glass), which is immersed in the solution. The immersed end of the body has a flat active or measuring surface and one or more light reflecting surfaces coated with a light-reflective material. The unimmersed end of the probe includes a light source for directing a beam of light rays through the light-transmissive body toward the measuring surface and a light sensor for receiving light rays reflected internally through the light-transmissive body from the reflective surface and providing an output signal indicative of the solution concentration. U.S. Pat. Nos. 3,917,411, 3,932,038, 3,977,790 and 4,037,967 are representative prior art references disclosing this type refractometric device. This type device usually has one or more of the following shortcomings: (1) can measure only a narrow range of concentrations, (2) the output is non-linear over the measured range, and (3) is expensive to manufacture because the employment of means for collimating the light (either transmitted or reflected), multiple reflecting surfaces, and/or other complicating features.